Medium frequency mine communication system

ABSTRACT

A method for using an underground mine communication system to effect minewide communication and an intrinsically safe current limiter circuit for insuring that electrical equipment in the system will not cause incendiary conditions. The underground mine communication system comprises a plurality of repeaters and medium frequency radios, including mobile, portable and personal-carried radios, coupled to electrical conductors and natural waveguides existing in the earth by tuned loop antennas. Messages transmitted by the radios are carried to the repeaters by the conductors or coal seam waves. The repeaters amplify, replicate and retransmit the message at two different frequencies for transmission of the message to a surface base station and to other radios in the system. A paging system, which has a separate set of repeaters, is also coupled to the network of electrical conductors and natural waveguides by tuned loop antennas. The paging system alerts miners to contact the surface base station. Radios, pagers and repeaters in the system are equipped with the intrinsically safe current limiter circuit to preclude the development of incendiary conditions. The current limiter circuit comprises a series arrangement of a current trip circuit, a redundant current trip circuit and a current limiting field effect transistor controlled by a feedback control amplifier.

This is a continuation of copending application(s) Ser. No. 07/389,403filed on 08/04/89 now abandoned which is a divisional of U.S. Ser. No.07/056,559 filed 5/29/87 now U.S. Pat. No. 4,879,755, issued 11/7/89.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a communication system for use inunderground mines and more particularly to a mine communication systemwhich includes a network of medium frequency transceivers anddouble-unit repeaters. The transceivers are magnetically coupled toelectrical and natural waveguide conductors within the mine by loopantennas and are protected from incinerary conditions by anintrinsically safe current limiter circuit.

2. Description of the Prior Art

It has long been known that medium frequency (MF) electromagnetic wavespropagate through natural media, such as coal and rock, as well asthrough electrical conductors such as track, wire rope and electricalwiring that exist in underground mines. Efforts have been made toexploit the propagation properties of MF signals to develop "wireless"communication systems. A wireless underground mine communication systemwould improve both mine productivity and mine safety. In a 1980 paper,Larry G. Stolarczyk proposed such a system. His cellular minecommunication system exploited both the conductor mode of transmissionand the natural waveguide mode of transmission for effecting MF radiocommunication within the mine. The system utilized a cellular repeaterto provide a means for two mobile transceivers to communicate with eachother. A communication link to the surface and to other repeaters wasprovided by a two-wire transmission line (i.e., a telephone line) overwhich voice signals in the audio frequency range were transmitted. L.Stolarczyk, The Design of a Cellular MF Radio Communication System forUnderground Mining, Reprint from National Telecommunications Conference(Nov. 30-Dec. 4, 1980). Thus, this system suffered from the inability ofsurface stations and repeaters within the system to communicate witheach other using radio frequency signals. This would become a seriousproblem if the telephone line was severed during a mine disaster, forexample. This early system, which included a transceiver and a loopantenna attached to a vest worn by a miner, was described in more detailby L. Stolarczyk and R. Chufo in System Design and Performance of an MFRadio Communication System for Underground Mining, (Sept. 1981). Theloop antennas used in these early systems were second order tuned loopantennas.

More recently, the MF wireless communicationn technique has beenexpanded to include a radio communication system which provides for theradio control of a mine train-loading operation. H. Dobroski and L.Stolardczyk, Control and Monitoring via Medium-Frequency Techniques andExisting Mine Conductors, IEEE Transactions on Industry Applications,Vol. 1A-21, No. 4 (July/August 1985). In this system, MF radio signalsare induced on existing conductors through the use of air core linecouplers.

Many attempts have also been made at using ferrite couplers to induce MFradio signals on conductors. These suffer from the problem that nosingle ferrite material functions satisfactorily as both a receiving anda transmitting line coupler.

Finally, surveys have been published which review the attempts atdeveloping various types of loop antennas for use in wireless minecommunication systems. R. Lagace, D. Curtis, J. Foulkes and J. Rothery,Transmit Antennas for Portable VLF to MF Wireless Mine Communications,USMB Contract Final Report (H0346045), Task C, Task Order No. 1 (ArthurD. Little, Inc. ) May 1977.

Loop antennas for use in mine communication systems that have beenincorporated into a bandolier type garment have long been known. See,e.g. B.A. Austin and G.P. Lambert, An Interim Report on the RadioCommunication System Installed Underground at Greenside Collery, ApexMines Limited, Chamber of Mines of South Africa Research Report No.39/77, Project No. CS1C10 (1977). The bandolier design suffers from thefact that as the miner's chest moves, the loop area of the antennachanges, thus changing the area and inductance of the antenna.

A separate direction in which mine communication methodology hasdeveloped is that of emergency communications. Two types of emergencymine communication systems are the seismic method and the boreholemethod. In the seismic method, a trapped miner transmits seismicvibrations by pounding on a rail or roof bolt. These signals aredetected by surface geophones. After computer analysis of the arrivaltimes of the seismic signals, the location of the trapped miner can bedetermined. The seismic method has proven inadequate because thedeployment of geophone arrays is time consuming and voice communicationis impossible. Additionally, the technique requires that the miner notbe seriously injured so that he can pound on a rail or roof bolt inorder to be detected.

in the borehold method, probes are lowered down boreholes in order toprovide two-way voice communications with trapped miners. This method isnot satisfactory because set-up drilling is time consuming and uselessif the exact location of a trapped miner is not known.

A safety requirement for all electrical equipment used in mines is thatthe equipment be intrinsically safe. Intrinsically safe equipment isincapable of releasing sufficient electrical or thermal energy, undernormal or abnormal conditions, to cause ignition of a specific hazardousatmosphere mixture in its most easily ignited concentration. IEEEStandard Dictionary of Electrical and Electronics Terms, 3rd Edition, p.463 (1984). To satisfy this requirement, the Mine Safety and HealthAdministration (MSHA) and the U.K. Health and Safety Executive (HSE)require that batteries be protected with a fuse and series resistorcircuit. The fuse in designed to blow out before the temperature of theresistor reaches a certain temperature. A disadvantage of this circuitis that it requires that larger batteries be used to compensate for thevoltage drop across the resistor. The use of larger batteries increasesthe size of mine equipment and decreases the capacity of the batteries.

SUMMARY OF THE PRESENT INVENTION

It is therefore an object of the present invention to provide anunderground mine communication system in which the plurality ofrepeaters and surface base stations can communicate with each otherusing medium frequency (MF) radio signals.

It is another object of the present invention to provide a method forcommunicating in an underground mine communication system whereby MFradio signals can be transmitted long distances to a surface basestation.

It is another object of the present invention to provide an improvedmethod for inductively coupling MF signals onto electrical conductors.

It is another object of the present invention to provide a method forinductively coupling MF signals onto natural waveguides.

It is another object of the present invention to provide an improvedcurrent limited circuit which prevents incendiary conditions fromdeveloping in radios within the underground mine communication system.

It is another object of the present invention to provide a transmitterfor use within the mine communication system that is optimized formaximum efficiency in generating a loop magnetic moment.

It is another object of the present invention to provide a portableradio for use within the communication system that can be carried byminers in emergency situations.

It is another object of the present invention to provide a personalcarried radio for use within the communication system that can be usedwith a plurality of antennas.

It is another object of the present invention to provide a paging systemfor calling or warning miners.

It is another object of the present invention to provide a vertical loopantenna unit for use within the communication system that can be worn bya miner and in which the loop area will remain constant.

It is another object of the present invention to provide a mobilehorizontal loop antenna for use within the communication system that canbe mounted on a vehicle.

Briefly, the present invention includes a method for using anunderground mine communication system to effect minewide communicationsand an intrinsically safe current limiter circuit for insuring thatelectrical equipment in the system will not cause incendiary conditions.The system includes portable radios, personal-carried radios, pagers,mobile radios and surface station radios all linked by a backbonenetwork of repeaters. The repeaters are double transceivers tightlycoupled to electrical conductors existing in the mine. The pagers have aseparate repeater network. Messages transmitted by radios in the systemare inductively coupled onto the electrical conductors at one frequencyby vertical and horizontal loop antennas on the radios. The message isreceived by the repeaters which then amplify, replicate and transmit themessage at two different frequencies. This allows the original radiomessage to be transmitted long distances to the surface stations as wellas to other radios in the systems. The pagers include a signal lightwhich is activated by a digitally encoded signal sent from a pager basestation on the surface. This allows key personnel underground, to becontacted independently of the radio system. Each radio, pager andrepeater in the system is equipped with an intrinsically safe (IS)current limiter circuit which insures that a fault in the system willnot result in incendiary conditions. The IS current limiter circuitcomprises a series arrangement of a current trip circuit, a redundantcurrent trip circuit and a current limiting field effect transistor(FET). The current limiting FET is driven to a high resistance state bya feedback control amplifier whenever excessive current demand is drawnthrough the circuit. The current trip circuit then latches the system inan open position until the current draining fault is removed. A heatresponsive thermistor attached to the current limiting FET supplementsthe feed back control amplifier. The transmitter unit of each radio andrepeater in the system is designed for maximum efficienty in generatinga loop magnetic moment. This is accomplished by optimizing the ratio ofthe power dissipated in the vertical loop antenna to the bandwidth ofthe frequency modulated MF carrier signal. In the present invention,thirty inch diameter loop antennas are used with the portable radios andrepeaters. The personal-carried radio can be used with a plurality ofloop antenna designs including a loop antenna which is incorporated intoa suspenders/belt combination which can be worn by a miner. The mobileradio utilizes a horizontal loop antenna sandwiched between two boardsand mounted on the outside of a mine vehicle.

An advantage of the present invention is that the repeaters cancommunicate with the base stations using MF radio signals.

Another advantage of the present invention is that MF radio signals canbe transmitted over long distances from a radio located remotely to anelectrical conductor to a surface base station.

Another advantage of the present invention is that vertical andhorizontal loop antennas are used for inductively coupling MF radiosignals onto electrical conductors.

Another advantage of the present invention is that the vertical loopantennas also inductively couple the MF radio signals onto naturalwaveguides.

Another advantage of the present invention is that an improved currentlimiter circuit is used to prevent incinerary conditions from developingin radios within the mine communication system.

Another advantage of the present invention is that the radiotransmitters are optimized for generating a loop magnetic moment.

Another advantage of the present invention is that it includes aportable radio that can be used during mining emergencies.

Another advantage of the present invention is that it includes apersonal-carried radio that can be used with a plurality of tunedvertical loop antennas.

Another advantage of the present invention is that it includes a pagingsystem for calling or warning miners.

Another advantage of the present invention is that it includes animproved tuned vertical loop antenna that can be worn by a miner andwhich maintains a constant loop area.

Another advantage of the present invention is that it includes a tunedhorizontal loop antenna that can be mounted on the outside of a vehicle.

These and other objects and advantages of the present invention will nodoubt become obvious to those of ordinary skill in the art after havingread the following detailed description of the preferred embodimentwhich is illustrated in the various drawing figures.

IN THE DRAWINGS

FIG. 1 is an idealized view of a medium frequency mine communicationsystem of the present invention;

FIG. 2 is a block diagram of a portable radio and vertical loop antennaof the communication system of FIG. 1;

FIG. 3 is a diagram of a suspender loop antenna and a personal-carriedradio of the communication system of FIG. 1;

FIG. 3b shows an alternative embodiment of the suspender loop antenna ofFIG. 3a;

FIG. 4 shows the pager of the communication system of FIG. 1;

FIG. 5 is a diagram of the mobile vehicular radio and mobile horizontalloop antenna of the communication system of FIG. 1;

FIG. 6a is a block diagram of a pair of repeaters of the presentinvention;

FIG. 6b is a block diagram of a pager repeater of the present invention;

FIG. 7a is a circuit diagram of a conventionally designed intrinsicallysafe battery protection circuit; and

FIG. 7b is a circuit diagram of a current limiter circuit of the presentinvention.

DETAILED DESCRIPTION OF THE PREFFERED EMBODIMENT

Referring now to FIG. 1, there is shown a medium frequency minecommunication system referred to by the general reference numeral 20.The communication system 20 comprises a portable radio 22, a mobilevehicular radio 24, a personal-carried radio 26, a pager 27, a pluralityof repeaters 28, a plurality of pager repeaters 29, a surface basestation 30 and a pager base station 31. It is understood that there maybe more or fewer than one of any of the radios 22, 24 or 26 and ofsurface base station 30, pager base station 31 or pager 27.

A plurality of transmission line electrical conductors 32 exist within amine 34. (The conductors 32 may be telephone cables, AC power cable,monitor cable, rails, steel pipelines, etc.) Magnetic coupling betweenthe transmission line electrical conductors 32 in an entry way 35enables radio signal current flow in one conductor to induce signalcurrent flow in a nearby conductor. The portable radio 22 is installedat a working face 36 of the mine 34 directly below the transmission lineelectrical conductor 32. A portable radio vertical loop antenna 38attached to radio 22 is held in close proximity to the electricalconductor 32 by a connector 40. The mobile vehicular radio 24 is mountedinside of a mine vehicle 42 and is connected to a vehicular horizontaltuned loop antenna 44, mounted on the outside of vehicle 42, via a cable46. The personal-carried radio 26 is a compact, battery poweredtransceiver designed to be mounted on a miner's belt. A personal-carriedvertical tuned loop antenna 47 worn by the miners, is connected to theradio 26. The pager 27 can also be carried by a miner and would also beconnected to the personal-carried vertical tuned loop antenna 47. Theplurality of repeaters 28 are transceiver units designed to receive twofrequencies F2 and F3 and to transmit two frequencies F1 and F3. Therepeaters 28 are located in close physical proximity to electricalconductor 32. The surface base station 30 includes a medium frequencytransceiver capable of transmitting the frequencies F2 and F3 andreceiving the frequencies F1 and F3. Base station 30 is located at themine surface portal or at any other central dispatch or monitoringpoint. The surface base station 30 and the pager base station 31 areindividually coupled to the plurality of transmission line electricalconductors 32 via a surface base station vertical loop antenna 49 and apager vertical loop antenna 50 which are located in close physicalproximity to conductors 32. The pager base station 31 includes a pagertransmitter 51, a pager encoder 52 and a pager computer 53.

Referring now to FIG. 2, there is shown the portable radio 22 in moredetail. The connector 40 is a nylon tie wrap which encircles theconductor 32 and the portable radio tuned loop antenna 38 holdingantenna 38 in close proximity to conductor 32. The antenna 38 is avertical tuned loop antenna having a diameter "d" of about thirtyinches, and is connected to the portable radio 22 through wire 54. Theradio 22 comprises a medium frequency (300-800 KHz) transceiver 55,capable of receiving one frequency F1 at a receiver 56 and transmittingtwo frequencies F1 and F2 from a transmitter 57; a stand-by battery pack58, protected by an intrinsically safe current limiter circuit 59; acharging and power regulation circuit 60; a speaker 62; a squelchcontrol knob 64; a noise cancelling microphone 66, a time-out circuit68, and an external speaker 69. A remote power supply unit 70 withintrinsically safe output, is connected to radio 22 through a cable 71.Under normal conditions, the portable radio 22 is mounted on a wall andserves as a stationary radio in the mine-wide communications system. Inemergencies, portable radio 22 can be disconnected from the power supplyunit 70 and conductor 32 and removed from its wall mount for portableuse.

FIG. 3a shows the personal-carried radio 26 and personal-carriedvertical tuned loop antenna 47 in more detail. The radio 26 is a small,approximately 17/8" thick×41/8" wide×8" tall, medium frequencytransceiver capable of transmitting two frequencies, F1 and F2, from atransmitter 80 and receiving one frequency, F1, at a receiver 82. Theradio 26 contains a squelch control knob 84 and is powered by a batterypack 86 which is equipped with an intrinsically safe current limitercircuit 88. The personal-carried tuned loop antenna 47 is a verticaltuned loop antenna that can be worn by a miner. In the preferredembodiment, the antenna 47 is a wire loop incorporated as part of asuspender loop antenna designated by the general reference numeral 92.The suspender loop antenna 92 is a one-piece harness comprising a pairof flexible shoulder straps 100 which loop over a miner's shoulders likesuspenders. A pair of cross-straps 104 run perpendicular to shoulderstraps 100, on the back of suspender loop antenna 92 (i.e., the pair ofcross-straps 104 would be situated on the miner's back). A space "w"exists between the two cross-straps 104. A plurality of slots 106, onthe lower ends of the shoulder straps 100, provide a means for securingthe suspender antenna 92 to a belt 107 worn around the miner's waist.The belt 107 could also be permanently attached to the suspender loopantenna 92. Alternatively, the slots 106 could be any other suitablemeans for securing the suspender antenna 92 to the belt 107 such as aplurality of buckles, snaps or buttons. The antenna 47 is attached tothe outside surface of the rectangle formed by the cross straps 104 andshoulder straps 100. A loop antenna tuning box 108 is securely fastenedto one of the shoulder straps 100. A series tuned circuit, locatedinside of tuning box 108, switches the antenna 47 between receiving andtransmitting modes. A connecting wire 109 connects radio 26 to thetuning box 108. An antenna plug 110 allows connecting wire 109 to beplugged into the tuning box 108. The antenna plug 110 also allows radio26 to be connected to antennas of other designs. A pair of clips 112attached to radio 26 provide a means for attaching radio 26 to anordinary belt worn around a miners waist.

FIG. 3b shows an alternative embodiment of the suspender loop antenna 92designated by the general reference numeral 114. Elements in suspenderloop antenna 114 which are analogous to elements in suspenders loopantenna 92 are designated by the original number followed by a primedesignation. In suspender loop antenna 114, a pair of cross straps 104'run perpendicular to shoulder straps 100' on the front face of thesuspender loop antenna 114. A wire loop antenna 47' is attached to theoutside surface of the rectangle formed the cross straps 104' andshoulder straps 100'. A loop antenna tuning box 108' is securelyfastened to a front surface of one of the shoulder straps 100'. Thepersonal-carried radio 26 is connected to the loop antenna tuning box108' by the connecting wire 109 and the antenna plug 110 in the samemanner as was described in FIG. 3a. Similarly, a plurality of slots 106'provide a means for securing suspender loop antenna 114 to a miner'sbelt as shown in FIG. 3a. The embodiment depicted as suspender loopantenna 114 is useful, for example, in mine rescue operations whererescue team members carry an oxygen tank on their backs. In thatsituation, the oxygen tank would detune the loop antenna if it were alsolocated on the miner's back.

FIG. 4 shows the pager 27 attached to the suspender loop antenna 92 ofFIG. 3a. The pager 27 is attached to a front surface of one of theshoulder straps 100 and comprises a receiver 116, capable of receivingthe frequency F4, a decoder 118 and a signal light 120. The pager 27 ispowered by a battery pack 122 which is protected by an intrinsicallysafe circuit 124. The pager 27 is connected to the loop antenna tuningbox 108 by a connecting wire 125 and an antenna plug 126. The antennaplug 126 allows the pager 48 to be connected to antennas of otherdesigns.

FIG. 5 shows the mobile vehicular radio 24 in more detail. Radio 24 is amedium frequency transceiver capable of receiving one frequency F1 at areceiver 130, and transmitting two frequencies, F1 and F2, from atransmitter 132. A squelch control knob 134 is located on the face ofradio 24. The radio 24 is mounted inside the cab of vehicle 42 and isprotected by an intrinsically safe limiter circuit 136. The cable 46links an antenna connector 137 to the vehicular tuned loop antenna 44.The antenna 44 is a long piece of wire fashioned into a rectangle lyinghorizontal to the bed of vehicle 42. The antenna 44 encircles aplurality of steel rods 138 coming up from the bed of vehicle 42. A pairof plywood boards 140 lie above and below antenna 44.

FIG. 6a shows the plurality of repeaters 28 in more detail. Eachrepeater 28 includes an access medium frequency transceiver 150 and alocal medium frequency transceiver 152. The access transceiver 150 iscapable of receiving a signal at frequency F2 at a receiver 154,amplifying and replicating the F2 signal at the frequency F3 andtransmitting the F3 signal from a transmitter 158. The local transceiver152 is capable of receiving a signal at frequency F3 at a receiver 166,amplifying and replicating the F3 signal at the frequency F1 andtransmitting the F1 signal from a transmitter 170. The receiver 154 istightly coupled to conductor 32 by a repeater vertical tuned loopantenna 174 and an antenna cable 176 which links antenna 174 to receiver154. The transmitter 158 is also tightly coupled to conductor 32 by arepeater vertical tuned loop antenna 180 and an antenna cable 181 whichlinks antenna 180 to transmitter 158. Similarly, receiver 166 andtransmitter 170 are tightly coupled to conductor 32 by a pair ofrepeater tuned loop antennas 182 and 183 respectively, and a pair ofantenna cables 184 and 186, respectively. The access transceiver 150 andthe local transceiver 152 are protected by a pair of intrinsically safe(IS) limiter circuits 192 and 196, respectively. A pair of sealed leadacid batteries 198 and 200 are connected to the IS circuits 192 and 196,respectively.

FIG. 6b shows one of the plurality of pager repeaters 29. Each repeater29 includes a transceiver 201 which comprises a receiver 202, capable ofreceiving the frequency F5, and a transmitter 204, capable oftransmitting the frequency F4. The receiver 202 is tightly coupled tothe transmission line conductor 32 by a pager repeater vertical tunedloop antenna 206 and an antenna cable 208 which links antenna 206 to areceiver 202. The transmitter 204 is also tightly coupled to thetransmission line conductor 32 by a pager repeater vertical tuned loopantenna 210 and an antenna cable 212 which links antenna 210 totransmitter 204. The transceiver 201 is powered by a sealed lead acidbattery 214 which is protected by an intrinsically safe limiter circuit216.

In the preferred embodiment of the present invention, the frequenciesF1, F2 and F3 are chosen to be 400 KHz, 520 KHz and 300 KHz,respectively. The basis for this choice is the empirical observationthat the optimal frequency for propagating signals in underground minetransmission line electrical conductors is 300 KHz. This is because theattenuation rate for electromagnetic signal propagating on thetransmission line electrical conductors, decreases with frequency ofpropagation. At 300 KHz the attenuation rate is only 2 dB/1000 ft.,whereas at 520 KHz, the attenuation rate is 4-5 dB/1000 ft.Additionally, for frequencies below 300 KHz the mine generatedelectrical noise increases by 6 dB for each "halving" of frequency.Thus, 300 KHz represents an optimal propagation frequency. In contrastto propagation efficiency on the transmission line electricalconductors, however, remote loop antenna to transmission line couplingimproves with frequency. Thus, 520 KHz signals are more efficientlycoupled between a remote antenna and a conductor than are 300 KHzsignals.

The functioning of the mine communication system 20 shown in FIGS. 1 and5 can now be explained. The portable radio 22, the mobile radio 24 andthe personal-carried radio 26 all use their respective tuned loopantennas 38, 44 and 47, to magnetically induce signal current flow innearby conductors 32. Because the antennas 38, 44 and 47 are often fourto fifteen feet from conductors 32 (remote), they induce only weaksignal currents in conductors 32. To increase the operating range of thesystem 20, the repeaters 28 are used to receive weak radio signals,amplify the signals and then reinduce stronger current flow in theconductors 32. For example, when communication from the mobile vehicularradio 24 is desired, a signal is transmitted at frequency F2. Thisfrequency allows efficient coupling between vehicular loop antenna 44and electrical conductor 32 even when they are not physically close toeach other. When the F2 signal, propagating in conductor 32, encountersone of the repeaters 28, the F2 signal is picked off by loop antenna 174and relayed to receiver 154. The F2 signal is amplified, replicated(i.e. changed to the frequency F3) and retransmitted by transmitter 158at frequency F3. Because antenna 180 is tightly coupled to conductor 32,the F3 signal is efficiently coupled back onto conductor 32 andpropagates to every repeater 28 in system 20 and to the base station 30.At every repeater 28, the F3 signal is received by the receiver 166 vialoop antenna 182. The F3 signal is then amplified, replicated andretransmitted at frequency F1 from transmitter 170 through loop antenna113 back onto conductor 32. Since every mobile vehicular radio 24,personal-carried radio 26 and portable radio 22 is always tuned tofrequency F1, they receive the signal.

The radios 22, 24 and 26 can also communicate directly with one another,at short range, without the use of repeaters 28, by transmittingdirectly on frequency F1.

The base station 30 can communicate with radios 22, 24 and 26 bytransmitting a message to the repeaters 28 on frequency F2. This messageis then replicated by the repeaters 28 and transmitted to the radios 22,24 and 26 on frequency F1. The base station 30 can also communicatethrough the repeaters 28 by transmitting on frequency F3 and receivingsignals at F1.

The pager 27 functions by alerting the person wearing the pager tocontact the surface. The pager computer 53, contained within pager basestation 31, can be programmed to initiate calls, periodically, until theperson wearing the pager is reached. The computer 53 would initiate thecall by generating a digital code, from the pager encoder 52, whichwould be modulated in a frequency shift key (FSK) format. The digitallycoded call would then be transmitted by the pager transmitter 51 at thefrequency F5. The call is transmitted from the pager loop antenna 50onto the transmission line electrical conductors 32 and to the pagerrepeaters 29. When the F5 signal, propagating in a conductor 32,encounters one of the repeaters 29, the F5 signal is picked off by loopantenna 206 and relayed to receiver 202. The F5 signal is amplified,replicated and retransmitted by transmitter 204 at frequency F4. Becauseantenna 210 is tightly coupled to conductor 32, the F4 signal is coupledback onto conductor 32. When a person wearing pager 27 and suspenderloop antenna 92 comes close to a conductor 32, the F4 signal is receivedby receiver 116, decoded by decoder 118 and used to activate the signallight 120. This alerts the pager wearer to contact the surface. Once thepager wearer has contacted the surface, the pager computer 53 isinstructed to cease sending calls. In the preferred embodiment, thefrequencies F4 and F5 are chosen to be 450 KHz and 250 KHz,respectively.

The use of tuned loop antennas (such as antennas 38, 44, 47, 174, 180,182, 183, 49 and 50 in the present invention) is important to thefunctioning of system 20 for three reasons. First, loop antennas arevery effective electromagnetic couplers in both the transmitting andreceiving modes. In the transmit mode, loop antennas produce highcurrent flow in nearby conductors and loop antennas do not changeinductance (saturate) when transmitting. Additionally, loop antennashave the capability of being either tightly electromagnetically coupledto a conductor (i.e. being coupled in close physical proximity to theconductor) or of being remotely electromagnetically coupled to aconductor (i.e. achieving coupling to a conductor even when the loopantenna is 1-20 feet away from the conductor). Second, loop antennashave the ability to couple both transmission line electrical conductorsand natural waveguides. Natural waveguides are formed when a layer ofless conductive material (such as coal, trong or potash) is boundedabove and below by more conductive rock. It is well known that theelectrical field component of an electromagnetic wave is verticallypolarized while the magnetic field component is horizontally polarized.Thus, in the mine 34, the loop antenna 38, hanging in a vertical planebelow conductor 32, can efficiently electromagnetically couple theelectrical conductor 32 and is also correctly positioned to receive themagnetic component of electromagnetic waves traveling in the naturalwaveguide mode in working face 36.

The importance of the natural waveguide coupling mode is that it enablescommunication links to be established through more than 1000 feet ofsolid coal and 300 feet of rock where no electrical conductors exist.Because of the ability of loop antennas to couple both electricalconductors and natural wave guide modes, the portable radio 22 wouldhave the following operating ranges in a mine 34:

    ______________________________________                                        Tight Coupling Operating Range                                                (Conductor Mode)                                                              Type of Conductor    Range (feet)                                             ______________________________________                                        Unshielded Wire Pair 33,000                                                   Shielded Wire Pair   20,000                                                   Remote Coupling Operation Range                                               Mode                 Distance (feet)                                          ______________________________________                                        Conductor             8,000                                                   Seam                  1,000 (radius)                                          ______________________________________                                    

Finally, the third functional advantage of loop antennas is that theyare easy to install and are much less expensive than other couplingdevices such as ferrite or air core torroid couplers. In field testing,it has been determined that the loop antenna 38 in FIG. 1 can besuspended from conductor 32 using a connector 40 which can be simply apiece of nylon string.

The design of the transmitters 57, 80, 132, 158, 170 and 204 in FIGS.2-6 is also important to the functioning of the present invention. Thetransmitters 57, 80, 132, 158, 170 and 204 are designed to yieldoptimization of the magnetic moment of the transmitting loop antenna.The magnetic moment (M) is given by the equation:

    M=NIA

where

N=the number of turns in the loop antenna;

A=the area of the loop antenna in square meters; and

I=the peak current in the loop.

Optimization of the transmitters is achieved by recognizing that for thetransmitting loop antenna, M=(P_(o) /BW)^(1/2) where P_(o) =the powerdissipated in the loop, and BW=the bandwidth of the FM carrier signal.So, in a series tuned circuit,

    Q=ωL/R.sub.L =P.sub.o /BW

where

Q=qualify factor (loaded Q)

ω=radian frequency

L=inductance (henry)

R_(L) =series resistance

To maximize the magnetic moment, the loop bandwidth (BW) is made assmall as possible while still being wide enough to accommodate theoccupied bandwidth of the FM carrier signal. Thus, in the presentinvention, the ratio P_(o) /BW is seen as the electrical optimizationparameter and not as just the maximum power to load resistance.

The receivers 56, 82 and 130 in FIGS. 2, 3 and 5 have been designed toinclude a means for measuring the received signal to noise (S/N) ratio.The squelch control knobs 64, 84 and 134 on the radios 22, 26 and 24 arecalibrated so that each click of the squelch control knob indicates achange in the S/N ratio by 10 dB. This feature circumvents the necessityof having radio maintenance personnel carry field strength meters todetermine the high voice quality communication range (fade margin).

The portable radio 22, shown in FIG. 2, serves two important functionsin the mine communication system 20. First, under normal conditions,radio 22 functions as a stationary radio transceiver in system 20. Aplurality of radios 22 would be placed in various locations in a mine 34such as at the working face 36 and in saferooms, belt loading points andcentral control or communication points. The external speaker 69provides a high volume audio capability so messages can be heard in thenear vicinity of the unit.

Second, under emergency situations, such as a fire, explosion orcave-in, the portable radio 22 can be removed from its wall mount andcarried by a miner enabling him to receive evacuation instructions andinformation.

Referring now to FIG. 7a, there is shown a conventionally designedintrinsically safe (IS) battery protection circuit 220. Circuit 220includes a wirewound resistor 222 and a fuse 224 connected in serieswith a battery 226. A pair of contacts 228 and 229 provide a means fordrawing current from circuit 220.

FIG. 7b shown an IS current limiter circuit 230 of the presentinvention. The circuit elements enclosed within the two dashed boxes ofFIG. 6b form a current trip circuit 232 and a redundant current tripcircuit 234. The circuit 232 includes a field effect transistor (FET)236 connected in series between a node 238 and a node 240. A branch ofnode 238 contains a resistor 242, a transistor 244 and an FET 246connected in series between node 238 and a ground 248 connected to thesource terminal of FET 246. The emitter of transistor 244 is grounded. Anode 250 is located between transistor 244 and FET 246. A resistor 252is connected between node 250 and the gate terminal of FET 236. A node254 is located between transistor 244 and node 250. A resistor 256 isconnected between node 254 and a anode lead 258 which is connected tothe anode of a twelve volt battery 260. A resistor 262 is connectedbetween node 240 and the gate terminal of FET 246. A node 264 is locatedbetween resistor 262 and FET 246. A resistor 266 is connected betweennode 264 and a ground 268.

The redundant trip circuit 234 has an electrical structure identical tothat of circuit 232 and includes a pair of FET's 270 and 272 and atransistor 274. The emitter of transistor 274 is grounded. The FET 270is connected in series between the node 240 and a node 278. A lead 279is connected between a node 280, which lies between resistor 242 andnode 238, and a node 281. A branch of node 281 contains a resistor 282,the transistor 274 and the FET 272 connected in series between node 281and a ground 283 connected to the source terminal of FET 272. A node 284is located between transistor 274 and FET 272. A resistor 286 isconnected between node 284 and the gate terminal of FET 270. A node 288is located between transistor 274 and node 284. A resistor 290 isconnected between node 288 and the anode lead 258. A resistor 292 isconnected between node 278 and the gate terminal of FET 272. A node 294is located between resistor 292 and FET 272. A resistor 296 is connectedbetween node 294 and a ground 298.

The area to the right of redundant trip circuit 234 in FIG. 6b includesa current limiting FET 300 connected in series between the node 278 anda wire-wound inductor 302. A feedback control operational amplifier 304is connected to the anode lead 258 at a node 305 by an output lead 306.A resistor 307 is connected in series between the node 305 and amplifier304. The amplifier 304 includes a current limit voltage comparactor 312and a case temperature limiter 314. An output lead 316 forms arectangular loop 318 which is connected between comparator 312 and thenode 281. The loop 318 includes a resistor 322 connected in seriesbetween node 281 and comparator 312. An input lead 330 of comparator 312is connected to a grounded resistor 332. Another input lead 334 ofamplifier 304 is grounded. An input lead 336 of the amplifier 304, whichincludes a resistor 338, is connected to the anode lead 258. A node 344is located on input lead 330 between grounded resistor 332 andcomparator 312. A lead 346, which includes a resistor 348, runs fromnode 344 to anode lead 258. An input lead 349 connects limiter 314 withlead 346 at a node 350. A lead 352, which includes a resistor 354,connects limiter 314 with anode lead 258. A node 356 is located on lead352 between limiter 314 and resistor 354. A grounded heat responsivethermistor 358, located near the current limiting FET 300, is connectedto lead 352 at node 356. A thermal connection 359 is made betweenthermistor 248 and FET 300 using thermally conductive epoxy. The gateterminal of an FET 360 is connected to the lead 306 at a node 361. Thedrain terminal of FET 360 is connected to the anode lead 258 at a node362 by a lead 363. The lead 363 includes a resistor 364. A ground 365 isconnected to the source terminal of FET 360. An output lead 366 isconnected between the limiter 314 and a node 367 lying on lead 363. Aresistor 368 is connected between the gate terminal of FET 300 and anode 369 lying on lead 366. A grounded capacitor 372 is connected tolead 346 at a node 374 lying between node 344 and node 350. An anodeterminal 380 is located at the end of anode lead 258 furthest removedfrom battery 260. A cathode terminal 382 is located at the free end ofinductor 302. The terminals 380 and 382 provide a means for connectingelectronic equipment to the circuit 130.

The area to the left of current trip circuit 232 in FIG. 6b includes acathode lead 384 which is connected to the cathode of battery 260. Aplurality of battery charging diodes 386, connected in series, joincathode lead 384 at a node 388 near the cathode of battery 260. Cathodelead 384 branches at a node 390. One branch of cathode lead 384 isconnected to a sense resistor 392. Along the other branch, a precisionwirewound resistor 394 is connected between node 390 and node 238. Asense connector 396 is located at the free end of resistor 392. Acathode charge connector 398 is located at the free end of the series ofdiodes 386. An anode charge connector 400 is located on the free end ofa lead 402 which connects to anode lead 258 at a node 404 near the anodeof battery 260.

The functioning of the IS current limiter circuit 230 of the presentinvention can now be explained. The current limiter circuit 230 isdesigned to replace the conventional IS battery protection circuit 220shown in FIG. 6a. The circuit 230 would be used with radios 22 and 24and repeaters 28 of FIG. 1. For example, FIG. 2 shows the portable radio22 equipped with a battery pack 58 and an intrinsically safe limitercircuit 59. The personal-carried radio 26 is equipped with a smaller ISlimiter circuit 88 having the same design as circuit 230.

The circuits 59, 136, 88, 192, 196, 216 and 214 are needed when usingthe radios 22, 24 and 26 and repeaters 28 and 29 and the pager 27 ingaseous atmospheres, such as are found in coal mines, to preventexplosions. The current trip circuit 232, shown in FIG. 6b, emulates thefuse 224 of FIG. 6a. The feedback control operational amplifier 304 inFIG. 6b emulates the resistor 222 in FIG. 6a. The redundant trip circuit234 of FIG. 6b serves as a back-up to current trip circuit 132. Theoperation of current limiter circuit 230 limits the instantaneous demandcurrent flow to an intrinsically safe level.

The initial condition of the FET's 236, 270 and 300 is a low channelresistance condition of about 0.18 ohms. The current flow throughresistor 394 produces a voltage V(1) by Ohm's Law. The current limitvoltage comparator 312 has a reference voltage V(2), which is normallygreater than voltage V(1), established by the biasing resistors 348 and332. As long as the voltage V(1) remains less than voltage V(2), thechannel resistance of FET 300 remains at the low value of about 0.18ohms. However, in the event an excessive demand of current flow iscaused by a fault in the equipment connected between terminals 380 and382 or in the current limit voltage comparator 312, the voltage V(1)rises above voltage V(2). This drives the output lead 306 to a low levelcausing the channel resistance of FET 300 to increase and thus limitingthe current flow through FET 300. Transistor 244 then turns on causingthe FET 236 channel resistance to increase. Simultaneously, the FET 246channel resistance goes to a low ohmic resistance state, for the purposeof latching FET 236 in its high resistance state, thereby permanentlyopening the demand current path. By opening terminals 380 and 382, thelatch condition in circuit 230 can be removed. The redundant tripcircuit 234 backs up the current trip circuit 232.

To further insure the fuse-like nature of circuit 230 and to preventoverheating of the FET 300, the heat responsive thermistor 358 isattached with thermally conductive epoxy to FET 300 at connection 359.If FET 300 heats up, the temperature increase is transferred tothermistor 358. Whenever the temperature of thermistor 358 exceeds alimit set by V(2), the gate of FET 300 is driven to a low state, thusincreasing the channel resistance of FET 300. The function of casetemperature limiter 314 is to prevent excess heat build-up in FET 300which could cause incendiary conditions to develop.

When equipment is connected across the terminals 380 and 382, atransient demand current flows to charge capacitors in the equipment.This current is slowed down by the wire-wound inductor 302. The energyof this current transient is limited to less than 0.2 millijoules.

The plurality of diodes 386 and the charge connectors 388 and 400provide a means for recharging battery 260. The sense resistor 392 andthe sense connector 396 provide a means for determining the batterycharging states. Initially, this is a high charge current rate followedby a flat charge rate.

The design of IS current limiter circuit 230 insures that a fault in anycircuit (such as a short circuit) will not cause incinerary conditionsto occur in the circuit. The entire circuit 230 is potted to preventcoal dust from accumulating on the component parts.

Although the present invention has been described in terms of thepresently preferred embodiment, it is to be understood that suchdisclosure is not to be interpreted as limiting. Various alterations andmodifications will no doubt become apparent to those skilled in the artafter having read the above disclosure. Accordingly, it is intended thatthe appended claims be interpreted as covering all alterations andmodifications as fall within the true spirit and scope of the invention.

We claim:
 1. A personal-carried magnetic dipole formed by a verticalplane loop antenna assembly for an underground communication systemincluding a transmission line electrical conductor extendinglongitudinally within a mine, the antenna assembly comprising:aone-piece harness for mounting on a person located within a mine, theharness including a pair of flexible shoulder straps which loop over aperson's shoulders, means for attaching the shoulder straps to a beltworn about the person's waist and a pair of cross straps which runperpendicular to the shoulder straps, along a back face of the harness,with each of the cross straps being connected to each of the shoulderstraps and a space "w" being left between the two cross straps such thatthe two cross straps and shoulder straps form a constant rectangulararea when the harness is mounted on a person; a tuned resonantcontinuous wire loop antenna attached to the outer face of the constantrectangular area formed by the cross straps and the shoulder straps toestablish a magnetic dipole loop antenna in a plane substantiallyvertical relative to said transmission line electrical conductor withinsaid mine such that the loop area establishes a horizontal electricfield for coupling to said conductor when the harness is mounted on aperson within said mine and proximate to said mine transmission line;and a magnetic dipole loop antenna tuning box electrically connected tothe wire loop antenna and attached to one of the shoulder straps, thebox being tunable to frequencies lying in the medium frequency range. 2.The personal-carried vertical tuned loop antenna of claim 1, wherein,thepair of cross straps run perpendicular to the shoulder straps along afront face of the harness.
 3. The personal-carried vertical tuned loopantenna of claim 1, wherein,the loop antenna tuning box contains aseries tuned circuit for switching the continuous wire loop between areceiving mode and a transmitting mode.
 4. The personal-carried verticaltuned loop antenna of claim 1 further including :a pager electricallyconnected to the loop antenna tuning box, said pager including areceiver and a means for alerting the person that a signal is beingreceived by the receiver. b. adjusting the loop bandwidth to thesmallest value possible that will still accommodate the bandwidth of thefrequency modulated carrier signal.